Musnad Ahmad 6:6

Ibrahim ibn Mahdi narrated to us, Salih ibn Umar narrated to us, from Mutarrif, from al-Sha’bi, from Yahya ibn Talhah ibn Ubayd Allah, from his father: That Umar saw him looking sorrowful, so he said: "What is the matter with you, O Abu Muhammad, that you are sorrowful? Perhaps the leadership of your cousin—meaning Abu Bakr—has distressed you?" He said: "No." And he praised Abu Bakr, but he said: "I heard the Prophet, peace and blessings of Allah be upon him, say: 'A word that no servant says at the time of his death except that Allah relieves his distress and brightens his complexion.' Nothing prevented me from asking him about it except the ability to do so, until he died." Umar said to him: "I indeed know it." Talhah said to him: "And what is it?" Umar said to him: "Do you know of a word that is greater than the word he commanded his uncle with? 'La ilaha illa Allah' (There is no god but Allah)." Talhah said: "It is, by Allah, it."